1. Field of the Invention
The present invention relates to methods for manufacturing display devices for displaying an image by allowing fine particles to migrate. In particular, the present invention relates to a method for forming a sealing layer for hermetically sealing hollows, present on a substrate, for storing a dispersion containing such fine particles and a dispersion medium.
2. Description of the Related Art
In recent years, various display devices for displaying an image by controlling the distribution of fine particles contained in spaces have been proposed. Among these display devices is an electrophoretic display device which displays an image using fine charged electrophoretic particles that are dispersed in an insulating liquid and allowed to migrate by applying an electric field to the charged electrophoretic particles.
When the charged electrophoretic particles are dispersed in such an insulating liquid, they are apt to migrate in the in-plane direction of a substrate. Since the migration of the charged electrophoretic particles from their predetermined positions causes a deterioration in a displayed image, regions where the charged electrophoretic particles can migrate must be limited.
In order to limit such regions where the charged electrophoretic particles can migrate, the following technique has been proposed: the charged electrophoretic particles and the insulating liquid are packed in a plurality of fine hollows, present on a substrate, defined by the substrate, partitions arranged on the substrate, and a face of a support layer placed on the partitions, the support layer face being opposed to the substrate and covered with a sealing layer, the partitions having end portions that abut the support layer and are covered with a sealing layer. If the hollows are completely sealed, the charged electrophoretic particles can be confined in the hollows.
A method for manufacturing an electrophoretic display device having such a configuration will now be described.
Partitions are provided on a substrate by an etching process, an ink jet process, or an embossing process, whereby separated cells are formed. The etching process corresponds to a known photolithographic process by which such partitions are formed in such a manner that a photosensitive resist film is exposed through a mask and the resulting film is developed and then rinsed. For the ink jet process, a material for forming the partitions is gradually deposited on the substrate and then cured, whereby the partitions are formed. For the embossing process, depressions are formed in a flat plate by pressing a template against the flat plate, whereby the partitions are formed.
After the partitions are formed as described above, a mixture (hereinafter referred to as a dispersion system) of charged electrophoretic particles and an insulating liquid is packed in the cells separated from each other by the partitions. A sealing material is placed on the resulting dispersion system and then cured, whereby the dispersion system is confined in the separated cells.
A counter substrate, which is a counter part of the substrate having the partitions thereon, is joined to the cured sealing material, whereby the electrophoretic display device is obtained. The sealing material used to prepare the electrophoretic display device must not be compatible with the dispersion medium.
In another method for manufacturing the electrophoretic display device, a mixture of a dispersion system and a precursor of a sealing material is packed in separated cells by an ink jet process. The sealing material precursor is incompatible with a dispersion medium of the dispersion system and has a density less than that of the dispersion system. In this method, after partitions are formed, the sealing material precursor is separated from the dispersion system and then finally placed above the dispersion system.
After such a configuration is obtained, the sealing material precursor is cured with, for example, ultraviolet light, whereby the sealing layer is formed. This allows the dispersion system to be confined in the separated cells. A counter substrate, which is a counter part of a substrate having the partitions thereon, is joined to the resulting sealing layer, whereby the electrophoretic display device is obtained (see Japanese Patent Laid-Open No. 2001-343672 hereinafter referred to as Patent Document 1).
However, the method disclosed in Patent Document 1 has two problems described below.
The first problem is that only a limited group of materials for forming the sealing layer is possible.
The cause of this lack of freedom in the selection of materials for forming the sealing layer can be roughly divided into two factors. One of the factors is that the density of the sealing material is limited. That is; in the known method, the sealing material must have a density less than that of the dispersion system. When the dispersion system contains an isoparaffinic solvent widely used as a dispersion medium, the sealing material must have a density less than one. However, most curable materials have a density greater than one; hence, a curable material with a density of less than one is not readily available.
The other factor is that the sealing material must be curable even in the presence of oxygen because the sealing material must cured in such a manner that the sealing material is in contact with air. However, an inexpensive, soft, ultraviolet-curable resin, such as an acrylate ultraviolet-curable resin, useful in forming a sealing layer is generally cured in the absence of oxygen. If an oxygen-free atmosphere is employed, however, a special system for removing oxygen from an atmosphere in which the sealing material is cured, must be used. These factors seriously reduce the degree of freedom in selecting the sealing material.
The second problem is that it is very difficult to uniformly form a sealing layer with a wide area.
That is because the sealing layer exposed to air has nonuniform portions. The non-uniform portions are formed because the sealing material forms droplets. In general, it is very hard to prevent the nonuniform portions from being formed. An increase in device size makes this problem more serious.